An extra complication is that, as a casual OS X user, it seems to me like OS X terminal emulators and tools are more likely to munge with the path

This might be a function of the casualness rather than the tools. Here's a thing to check out on OS X:

    man path_helper 

I didn't know about this for ages and it makes your path life much saner - at least you'll have some idea where seemingly magical paths come from. Personally, I use it for everything and don't let other profile scripts eff with the global path at all.